The present invention relates to a method for operating a forward-guiding driver assist system, in particular an ACC (automatic cruise control) system, in a motor vehicle, and to a motor vehicle with such forward-guiding driver assist system.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Forward-guiding driver assist systems in motor vehicles are known in the art. One type of configuration is implemented as a so-called cruise control system, wherein the driver can select a desired speed which the driver assist system then controls by way of braking and acceleration interventions. So-called ACC (adaptive cruise control) systems with two operating modes have been proposed to allow a forward-guiding driver assist system to also be used for distance-controlled driving. In an unrestricted driving mode, the desired speed is controlled in the same way as known from cruise control. However, if a vehicle in front moves slower than the desired speed, the ACC system is operated in a slave mode, whereby a certain distance, mostly a time gap, to the vehicle in front as controlling object is regulated through braking and acceleration interventions. The controlled distance can also be designed so as to allow modifications by the driver.
Such forward-guiding driver assist systems, in particular ACC systems, were also proposed for motor vehicles with a manual transmission. In motor vehicles with a manual transmission, the driver manually operates the clutch of the manual transmission via the clutch pedal operating as an actuator. ACC systems in motor vehicles with manual transmissions presume that a valid forward gear is engaged and the drive train, including the clutch, is fully engaged, representing a coupled mode. Otherwise, the ACC function is deactivated or cannot be activated.
ACC systems, or more generally forward-guiding driver assist systems, frequently also use for controlling the speed of the motor vehicle an overrun cutoff for the engine so as to use the drag torque deceleration (engine brake). As soon as the engine of the motor vehicle transitions into the overrun cutoff, the motor vehicle is decelerated due to the applied drag deceleration, which may be different and quite noticeable depending on the engine variant and the engaged gear.
Conventional forward-guiding driver assist systems, in particular ACC systems used in motor vehicles with manual transmissions exhibit several problems. A first group of problems results because certain required target accelerations or target decelerations are not fully available. This will be described in more detail with reference to an example. In an exemplary motor vehicle, the drag torque deceleration is −0.8 m/s2, when the engine is in overrun cutoff mode. If the motor vehicle were to move freely with the clutch disengaged (coasting mode), the final vehicle acceleration would be 0 m/s2. Vehicle accelerations in a range >−0.2 m/s2 could be adjusted with the torques adjustable in driving mode. However, this means that a relatively large acceleration band exists in the coupled state, which cannot be controlled at all or is difficult to control.
Control of the forward-guiding driver assist systems frequently has the problem that although a slight deceleration or coasting of the motor vehicle should be realized, the overrun cutoff torque of the engine and hence the drag torque deceleration are too large, causing the motor vehicle to slowed down more quickly than actually desired by the forward-guiding driver assist system. As a result, the driver assist system is then forced to transmit a torque demand to the engine to terminate the overrun cutoff, because the speed should be reduced less quickly. Disadvantageously, this approach produces side effects which negatively affect the comfort. Switching into the drive mode is frequently noticeable and results in a “jerky” overall effect. Moreover, engine variants are known where small torque demands cannot be exactly set. The aforementioned “jerk effect” can then be quite pronounced and cause oscillations, because the engines frequently provide excessively high torques, which may then lead to an undesirable undue speed increase, so that the control function of the forward-guiding driver assist systems must then again severely reduce the torque demand or in extreme cases even apply the brake following the torque conversion. This may also cause intermittent operation of the brake light. An exceedingly fast change of the operating mode of the engine into or out of the overrun cutoff also reduces the comfort of the functionality of the forward-guiding driver assist system, because the effects are directly fed back to the motor vehicle via the drive train.
To eliminate these comfort-related problems, it has been proposed to use defined hystereses for controlling the engine and the brake within the control function of the forward-guiding driver assist system, which has the disadvantage that the desired speed cannot be cleanly regulated in these situations, or the desired distance or the controlled distance cannot be exactly maintained.
Disadvantageously, forward-guiding driver assist systems presently employed in motor vehicles with manual transmissions are typically unable to implement measures for optimizing fuel consumption, for example use of a coasting mode and the like. To operate the forward-guiding driver assist system, a valid forward gear stage must be engaged and the drive train must be completely closed, meaning that the clutch must be engaged. This approach prevents a driving strategy of the driver assist system for optimizing fuel consumption, where the vehicle coasts freely with an uncoupled drive train or drives with clutch slip.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved forward-guiding driver assist system for use with manual transmissions which improves driving comfort and energy consumption.